This brief comment became the biggest headline news to come out of the third debate, as many saw it as Mr Trump threatening to shatter a 240-year-old electoral tradition, one of the cornerstones of US democracy: the losing candidate must always concede defeat, regardless of the result.

Presidential rival Hillary Clinton called his stance “horrifying”, saying it “was not the way our democracy works”.

It was a moment that stirred a question in the minds of voters: what if Mr Trump decided to interfere with the result and mount a legal challenge, egged on by his discontented supporters?

The Independent spoke to Professor Iwan Morgan, director of the American Presidency Centre at University College London, and Thomas Donnelly, the National Constitution Center’s Senior Fellow for Constitutional Studies, to see if it was a realistic prospect.

What would happen if Donald Trump launched a challenge?

Based on his recent unfounded accusations that this will be a “rigged” election, Mr Trump could decide to launch a legal challenge in the event the result came down to a small number of votes in a few key states.

“Suppose Clinton wins with less than 50 per cent of the vote, which is quite possible with third-party candidates taking votes away from the two leading candidates,” Professor Morgan said.

“Trump would argue she’s not the true ‘president of the people’ and assemble lawyers for a challenge in the Supreme Court.

“It would be a completely new issue for the American political system to tackle, and would effectively tear up the electoral rulebook.”

Mr Trump would, in effect, be setting himself up as an alternative leader for his own supporters, who may then refuse to recognise the authority of their new president, and potentially create public unrest.

Mr Donnelly said a challenge could undermine the power of the president, presenting a difficult obstacle to the US political institution.

“Constitutionally-speaking, the free speech tradition we have in the United States allows candidates to criticise and challenge the result, as long as it doesn’t incite violence,” he said.

“But the peaceful transfer of power is a very important thing in American politics, dating back to our founding fathers.

“Even when there was litigation following the George Bush-Al Gore election [in 2001], shortly after we saw a very humble concession speech from Mr Gore, which ruled out any further difficulties. It’s just the way it happens.”

What are the legal requirements for a challenge?

Mr Trump would need “significant evidence” that voting fraud had been committed “on a large scale” on election day for his challenge to be successful, Professor Morgan told the Independent.

Otherwise, his attempt to dispute the result would have no legal grounding and would be swiftly quashed in the Supreme Court.

Mr Donnelly said this is backed up by US federal law: “They would have to challenge the voting results state-by-state, asking for recounts on close ties.

“Any challenger would have to allege violations of state and federal law in great detail, which could take some time. Very strong legal arguments would be required for a challenge to carry any weight.”

At present, there is no evidence for any illegal behaviour in the US electoral system. A study by the Loyola Law School found only 31 credible cases of voter fraud among more than one billion votes cast in elections from 2000 to 2014.

This despite Mr Trump claiming during a recent rally in Wisconsin that “voter fraud is very, very common”, and arguing voting booths could be “rigged” easily by people wishing to sabotage his campaign.

Has it happened before?

There is no modern precedent for challenging the result of a US presidential election, because the outcome has only been formally disputed twice in history, both times during the 19th century.

In 1824, the result was determined by the House of Representatives after neither candidate was able to secure enough votes. Critics of new president John Quincy Adams said he had secured his position through a “corrupt bargain”.

The 1876 election was one of the most contentious and controversial presidential elections in American history, with major disputes regarding vote counts in four southern states. An informal deal was struck to remove Republican troops from the south, in return for Rutherford Hayes becoming president.

“The closest we have come in modern times is when Eisenhower encouraged Richard Nixon to dispute the result of the 1960 election against Kennedy, because of the votes in Texas and Illinois were disputed,” Professor Morgan added.

“Nixon declined to challenge the result, because he was worried it might leave America without a president during a crucial period in the Cold War. So there are no modern examples we can look to for guidance.”

Could Hillary stop a potential challenge?

As mentioned before, there are no legal precedents to prevent Mr Trump from criticising and refusing to accept the result – so Mrs Clinton would have to deal with any challenges as they came.

“The only way to make the result completely indisputable, and to stamp out any challenge, is for Clinton to win big, by a landslide,” Professor Morgan continued. “But the odds are currently against that.”

The last time a presidential election was won by a landslide was in 1984, when the incumbent Ronald Reagan beat Walter Mondale to remain in power, with the approval of 49 states.

However, Mrs Clinton would not need the ‘blessing’ of her opponent take power, and the process of her becoming president could go ahead as usual without Mr Trump’s concession – even while a challenge was being discussed in the Supreme Court.

How long can Trump dispute the result?

Professor Morgan believes Mr Trump is likely to dispute the result “for months” if Mrs Clinton wins the election by a narrow margin, with “awful implications” for US electoral democracy.

“Trump will probably dispute the result well into 2017 if he loses. Once the US electoral college validates the result, that’s final,” he said .

“But it won’t do much to stop any public unrest which could result from Trump losing, given the quantity of Trump supporters who already think the vote for this election will be rigged.”

Mr Trump might even choose to launch another campaign in 2020 if he is unhappy with the result, Professor Morgan suggested.